The invention relates to an onboard positioning system, in particular, to an onboard positioning system which is used to provide an onboard determination of the position of a vehicle by utilizing radio waves from a positioning satellite such as GPS (Global Positioning System).
A technology for a positioning system for use on a vehicle while utilizing GPS is previously proposed, and is implemented in some form, as disclosed in Japanese Laid-Open Patent Applications No. 157,009/1986 and 167,886/1986, for example.
GPS satellite transmits a radio wave directed to the earth at a given timing which is modulated by data representing an exact timing, an orbital function and the accuracy of information. The location of the satellite can be determined from the timing and the orbital function while a difference between the satellite timing and the time at the point where the wave is received, which indicates a delay time required for the transmission of the wave, may be used to determine the distance from the satellite to the point of reception. If the positions of three different satellites and the distances from these satellites to the point of reception are known, the position of the point of reception in three dimensional space, which is unknown, can be determined by solving ternary simultaneous equations. However, the time at the point of reception normally contains an error of a relatively large magnitude, which must be compensated for by utilizing information from four satellites simultaneously. If it is assumed that the altitude of the point of reception remains substantially unchanged, it is also possible to determine the position of the point of reception in two dimensions such as latitude and longitude or around the surface of the earth from information obtained from three satellites.
However, the number of available GPS satellites is limited in actuality, and in addition, a vehicle may frequently stay or pass positions where the radio wave is shielded, as when entering a tunnel or running behind a building. Accordingly, a difficulty is experienced in maintaining the reception of radio waves from three or four satellites at all times. It then follows that the determination of the position by utilizing radio waves from the satellites is inhibited, unless information from three or more satellites are available simultaneously.
In Japanese Laid-Open Patent Application No. 137,009/1986, there is disclosed a technique in which an optical fiber gyro is used to detect a change in the azimuth or orientation of a vehicle while a vehicle speed sensor is used to determine a distance travelled by the vehicle to enable the detection of the position of the vehicle from the reception of a radio wave even from a single satellite through an auxiliary use of information detected in this manner. Japanese Laid-Open Patent Application No. 167,886/1986 discloses a technique in which the detection of the position of the vehicle is enabled from the reception of radio waves from two satellites through an auxiliary use of a magnetic azimuth instrument and a distance meter which are mounted onboard.
Japanese Laid-Open Patent Application No. 137,009/1986 determines the position in terms of the latitude and the longitude, basically assuming that the vehicle assumes a constant altitude, and also suggests that the altitude can be determined by utilizing an altimeter.
In the prior art described above, where the number of available satellites is insufficient, the distance travelled by the vehicle is determined from a signal delivered by a vehicle speed sensor to compensate for any wanting information. However, the vehicle speed sensor must be mounted at a given location adjacent to a wheel, which involves difficulty in mounting, and hence is undesirable to use.